1. Field of the Invention
The present invention is directed to providing a circularizer for use with a laser ouputting a non-circular beam. More particularly, the present invention is directed to a circularizer including an optical element having a high numerical aperture and an additional optical element, both of which are provided on a bench having features for receiving the optical elements therein.
2. Description of Related Art
Laser diodes are compact, efficient, inexpensive and ideal for mass production. This has lead to an increasing number of applications for which laser diodes are used. However, the gain region of the laser diodes is quite small and asymmetric. This results in a small and asymmetric resonator mode. Consequently, the beam divergence is severe and asymmetric.
Failure to reduce or eliminate the ellipticity of the beam is sometimes unimportant, but many applications require collimated beams with low optical aberrations. If the beam is to be collimated with a conventional lens for a particular application, the resulting beam dimensions will be different in the two directions, resulting in an elliptical beam. Thus, the ellipticity of the beam must be reduced by a subsequent optical system.
Astigmatism is a further property of laser diodes which presents an additional potential disadvantage in many applications. An uncorrected astigmatic laser beam cannot be collimated by a single radially symmetric lens. With an astigmatic laser, each axis appears to be diverging from a different axial point. Thus, collimation of both axes simultaneously with a single element requires an anamorphic element with different focal lengths for each axis. This astigmatism is often corrected most advantageously by a subsequent optical system.
There have been numerous design solutions for collimating, circularizing and correcting wavefronts of diode lasers. One such solution includes using conventional cylindrical lenses to collimate each axis independently. However, the performance of such cylindrical lenses is degraded for more asymmetric beams, since skew rays cannot be corrected, and is hard to align. Attempts to solve this difficulty in alignment are disclosed, for example, in U.S. Pat. No. 5,581,414 entitled "Microlens Assemblies and Couplers" to Snyder which mechanically establishes a fixed relationship between a pair of microlenses. A spacer positioned between the pair of microlenses includes a hole which acts as a hard aperture allowing light to pass between the lenses. The spacer provides a set spacing corresponding to the thickness of the spacer. The cylindrical lenses must be placed very close to the laser, i.e., within a few microns. Since alignment tolerances are proportional to the desired distance, such a small distance results in a very small or tight tolerance, i.e., sub-micron, which is very expensive to achieve. However, by having the collimating optics so close to the laser, the astigmatism of the laser may be ignored, since the beam will not have had time to spread appreciably.
Another solution uses cylindrical lenses in conjunction with an aspheric collimator as disclosed in U.S. Pat. No. 5,553,174 entitled "Monolithic Cylindrical Optic" to Snyder. The '174 patent characterized the problems with the solution of the '414 patent were indicated as including mounting the lenses in close proximity to the face of the laser diode, thereby requiring that a can housing the laser diode be opened or the laser diode be provided without the can. In the '174 patent, this problem is solved by placing an aspheric collimator in front of a window of the can containing the laser diode to collimate the beam in the fast axis. A following pulled cylindrical lens is then used to collimate and magnify the slow axis to provide a collimated beam which is nearly circular. However, the aspheric collimator is not very light efficient and the pulled cylindrical lens is difficult to make.
Another configuration is shown, for example, in U.S. Pat. No. 5,229,883 to Jackson et al. As can be seen in FIGS. 2 and 3 of Jackson et al., a cylindrical lens 22 is used in conjunction with a binary optical element 28. The binary optical element 28 is designed such that each ray of light from the diverging input light source will travel the same optical path length or vary from the optical path length by a discrete multiple of the wavelength of the light traveling from its source to its exit from the front surface of the binary optical element. While the low horizontal divergence may typically be collimated with cylindrical optical elements with few resulting aberrations, collimation of the fast vertical divergence requires optical elements with increased optical power at a much lower f number which generally results in significantly tighter alignment tolerances or increased optical aberrations with the collimated output.
The cylindrical lens 22 is used to collimate the laser diode's output in the fast axis. Binary optics 28 include a substrate on which a binary optical diffraction pattern is etched. The binary optic diffraction pattern is typically an eight phase level structure which corrects for optical path differences inherent in the divergent light. The binary optical element collimates the slow axis divergence and corrects for skew ray aberration of rays not in either the fast or slow axis. These additional corrections are readily achievable in a binary optical element whose diffraction pattern is chosen so as to have each optical ray travel in equal optical path lengths or an optical path length that varies from that equal optical path length by integer multiple of the wavelength of light traveling therethrough.
In order to provide proper correction of the beam output from the laser diode 10, the cylindrical lens 22 and the diffractive optical element 28 in the Jackson et al. configuration must be both properly positioned along the optical axis, but also rotationally aligned with one another. This rotation alignment is crucial and exacting, since the different axes are treated differently. This rotational alignment can be difficult and sensitive, requiring expensive continuous rotational alignment.
The above solutions are therefore either expensive due to tight tolerance requirements or not efficient at delivering light. The less expensive solutions are satisfactory for uses which do not require a lot of power from the laser to operate effectively. Thus, while the previous inexpensive solutions only provided approximately 30-40% of the laser power to the system, it was easy to make lasers of sufficiently high power to meet the requirements for the system. In other words, the high losses could be tolerated due to the availability of sufficiently powerfull lasers.
However, with the advent of technologies requiring higher powers to operate, e.g., digital video disks and flying head magneto-optic drivers, delivery of higher laser powers to the systems is required. These increased power requirements can no longer be met by simply providing a more powerful laser, so a more efficient, cheaper collector lens is required.
One solution which teaches using high numerical aperture lenses for collecting a majority of the laser power when circularizing and collimating a beam is set forth in U.S. Pat. No. 5,636,059 to Snyder entitled "Cylindrical Microlens External Cavity for Laser Diode Frequency Control.". The configuration disclosed in this patent uses two cylindrical lenses, each of which collimates a respective axis of the laser beam, and requires active alignment.
When two elements, both providing optical power, need to be aligned, often active alignment is required. Active alignment is performed by turning on a beam and continuously adjusting the optical elements until a desired beam is achieved. Such active alignment is difficult and expensive. When optical power is provided by both elements, optimal alignment of a single element alone may not provide satisfactory alignment with the other optical element to provide the optimum results for the system as a whole. Each element must be precisely aligned.